A major factor affecting the efficiency of an internal combustion engine is the compression ratio. In general, the higher the compression ratio, the more efficient the engine. Unfortunately, there are a number of considerations that impose limits on the maximum value that can be used. Generally, the compression ratio used is based on the one that provides satisfactory anti-knock operation over the anticipated useful power range, with particular attention to full power operation. At full power, the fuel and air mixture enters the cylinder at essentially ambient conditions. The compression raises the pressure and the temperature to a level that will not cause pre-ignition detonation and, after ignition, produces the desired peak pressure in the cylinder. When operating at less than full power, the adiabatic expansion, due to throttling, causes the fuel and air mixture entering the cylinder to be at less than ambient pressure and temperature. These conditions present an opportunity to use a higher compression ratio; in fact, it is possible to produce conditions, prior to and after ignition, similar to those obtained at full power. The desired reduction in power is realized by expanding a smaller volume of gas. The increased compression ratio would increase efficiency. Since most automobiles operate at less than full power most of the time, the ability to vary the compression ratio appropriately would improve efficiency for most operating conditions.
A large number of patents have been issued to vary the compression ratio. These have included various ways to change the length of the connecting rods or to utilize eccentric members on the crankshaft or connecting rods which, when rotated, change the compression ratio. None of these patents has resulted in a commercially viable product, largely due to complexity and uncertainty about their efficacy.